Flowers, pollination, seeds

A flower is the plant's reproductive structure. That's the whole thing. Most flowering plants rely on animals to carry pollen: weighting by how many species each climate zone holds, Ollerton and colleagues put the global share at about 87.5 percent, with surveyed communities averaging 78 percent in temperate zones and 94 percent in the tropics (Ollerton et al., 2011). Not every floral trait is there for a pollinator, though. One review sorts them into signals, which pollinators themselves selected for, and cues, which other pressures shaped but which pollinators can still learn to use (van der Kooi et al., 2023).

Four sets of parts

• Sepals: usually green, leaf-like parts at the base of the flower.
• Petals: usually the showy part that draws pollinators in. Botanists group recurring flower traits into pollination syndromes: brightly colored, scentless flowers that open by day tend to draw birds, and hummingbirds are built to reach the nectar of tubular ones; blue or yellow flowers with markings only bees can see tend to draw bees; pale, fragrant flowers that open at night tend to draw moths. Treat these as tendencies rather than rules. In one global test across six plant communities, where botanists recorded who actually visits, the nearest syndrome predicted the main pollinator for only about a third of species (Ollerton et al., 2009).
• Stamens (male): a filament topped by an anther that makes pollen.
• Pistil (female): a stigma, a style, and an ovary holding the ovules.

Petal Sepal Stamen Stigma Style Ovary

Fig. 11. Sepals and petals frame the reproductive parts. Stamens (male) make pollen; the pistil (female) is the stigma, style, and ovary, which holds the ovules that become seeds.

Pollination to seed

Pollination moves pollen from anther to stigma. The grain grows a tube down the style to an ovule, a sperm cell fuses with the egg (fertilization), the ovule becomes a seed, and the ovary often swells into a fruit that carries the seed away from the parent.

Why it matters for care

If you grow houseplants for their leaves, this part is background. Indoor edibles are the exception, and they differ more than you'd expect. Tomatoes release their pollen through small pores in the anthers, so a full crop probably depends on the flowers being shaken, by wind, by mechanical movement or by visiting insects (Cooley & Vallejo-Marín, 2021). Bumblebees do it by clamping their jaws onto the anther cone and vibrating their flight muscles fast, which shakes the pollen loose (Brust, 2023). Honeybees have never been recorded vibrating flowers to collect pollen (Vallejo-Marín, 2022). Without bees, growers shake the flowers themselves: extension guidance for small greenhouses is to touch a running battery toothbrush to the stem of each tomato flower cluster, not the flowers themselves, for a few seconds every other day; touching the bloom can leave a hole in the fruit as it forms (Pundt, 2017). Mississippi State Extension says a toothbrush will pollinate some flowers but gives lower yields than a purpose-built electric pollinator (Mississippi State University Extension Service, n.d.). Keep expectations modest either way: in a review of tomato studies, mechanical shaking did not significantly increase fruit weight over unpollinated flowers, while buzz-pollinating bees did (Cooley & Vallejo-Marín, 2021). Peppers can pollinate themselves, and most flowers will set fruit with no pollen from a second plant, but as with many nightshades the pollen has to be shaken out, by wind or buzzing bees, and in still air, as in a windless greenhouse, that job generally falls to insects (Seeds of Diversity Canada, n.d.). Strawberries will also set unassisted, but each of those little seeds on the outside is a separate fruit that has to be fertilized on its own. Next to a seed that goes unfertilized, which stays green, the flesh does not swell (Nishimoto et al., 2023). Fertilized seeds make the hormone auxin, which helps drive the fruit's growth (Klatt et al., 2014). Indoors, a soft brush set on the center of each open flower and circled for about five seconds copies what one indoor strawberry trial did with a small tuft of feathers (Nishimoto et al., 2023). And once a plant is setting seed and fruit, that's mainly where its sugar goes (Clark et al., 2018).